Segmented switched reluctance electric machine with interdigitated disk-type rotor and stator construction

ABSTRACT

The present invention relates to a a unipolar current electrical machine operable as a switched reluctance motor, comprising a motor structure and a control unit. The motor structure comprises a plurality of disk-shaped annular stator elements stacked and spaced equidistantly from each other and mounted on a frame; a plurality of disk-shaped rotor elements mounted on a rotational axis and spaced equidistantly from each other such that successive rotor elements are positioned between successive stator elements; a plurality of electrical windings on each of the stator elements, which when energized with a current flow, produce a magnetic field in a direction substantially parallel to the axis; and a return path for completing a magnetic flux path in a second direction perpendicular to the first direction on successive stator elements and through the rotor elements for generating a rotational force in the rotor elements; and a rotor position sensor. A rotor position sensor provides a signal to the control unit. The control unit utilizes an input signal in combination with the rotor position signal to switch current to each of the windings of the stator elements in a controlled manner.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.10/933,711, filed Sep. 3, 2004, now pending, which is a continuation ofapplication Ser. No. 10/459,358, filed Jun. 11, 2003, now U.S. Pat. No.6,803,847 B2, which is a divisional application of application Ser. No.10/077,278, filed Feb. 15, 2002, now U.S. Pat. No. 6,713,982 B2, whichclaims benefit of U.S. Provisional application 60/270,032, filed Feb.20, 2001 which are all incorporated herein by reference in theirentireties.

FIELD OF THE INVENTION

The present invention relates to a unipolar current electrical switchedreluctance machine which uses a segmented coil construction to maximizeactive surface area of magnetic flux between rotor and stator elements,resulting in higher efficiency of flux utilization, to a method ofoperating such a machine in a fault tolerant manner and to a method ofmanufacture of such a machine. In contrast with induction motors, whichutilize alternating current, a switched reluctance machine operates witha unipolar current flowing through its coils.

BACKGROUND OF THE INVENTION

Electric machines such as motors and generators, are generally usedbecause they are extremely rugged, reliable, easy to control, and inparticular have a high torque capacity and high power density ratings.Switched reluctance machines operate on the principle that currenttraveling in stationary coils or windings of a stator produces arotating magnetic field which in turn interacts with a rotor occupyingthe space where the rotating magnetic field exists. The magnetic teethof the rotor react with the rotating magnetic field to produce arotational force.

Heretofore, it was believed that there was a fundamental limit to torquedensity in such machines. Although flux density is limited by materialconsiderations, while current density is limited by (1) heating, (2)machine reactance, and (3) the fact that too much current densityproduces localized magnetic saturation, the present invention optimizesthe configuration of the rotor and stator elements so that the machineoutput can be increased without substantially increasing the volume ofthe machine. Conventional belief in the design of electric machines isthat power density is limited and the only way to increase power outputis to increase the volume of the machine.

Switched reluctance motors operate on the principle of unipolar current,i.e., the current flows only in one direction in the windings regardlessof whether positive or negative torque is required. This principlerequires only one switch to be in series with each winding in eachstator element. The turning on or off of this switch regulates the flowof current in the winding. It should be noted that in the motorliterature an individual winding of a stator element is sometimesgenerally referred to as a “phase”. In the context of a unipolar currentelectrical machine the term “phase” is somewhat analogous to a phase ofa multiphase alternating current motor.

The primary object of this invention is to provide a switched reluctanceelectrical machine having a high torque capacity for a given machinevolume. A second object of this invention is to provide such a switchedreluctance electrical machine in which the commonly accepted limit totorque density in electrical machines is overcome by utilizing the samemagnetic flux among one or more parallel air gaps. A third object ofthis invention is to provide a switched reluctance electrical machine inwhich the magnetic flux is passed through multiple air gaps, interactngwith a rotor element at each air gap, thereby increasing the torquedensity for a give volume of the machine. A fourth object of thisinvention to provide such an electrical machine in which force densityis increased substantially by the number of air gaps present in themachine but the overall machine transverse dimension is increased onlyby a smaller factor because the magnetic return path remains nearlyconstant. A fifth object of this invention is to provide a faulttolerant switched reluctance electrical machine which can continue tooperate even when one or more winding faults have been detected. A sixthobject of this invention is to provide a method of manufacture for sucha switched reluctance electrical machine.

BRIEF SUMMARY OF THE INVENTION

The present invention is a unipolar current electrical machine having ahousing, a stator mounted in the housing, and a rotor, the rotor havinga shaft with an axis therethrough and being supported by bearings forrotation about the axis in the housing, comprising:

the stator having a plurality of stator elements each in the form of anannular disk, spaced apart from each other, each stator elementcomprising a plurality of magnetically isolated magnetic teeth;

a plurality of electrical windings on each of the stator elements, eachwinding being associated with a group of magnetic teeth of the statorelement, each group of magnetic teeth being arranged at a predeterminedangular position with respect to an adjacent group of magnetic teeth,each of the windings being arranged such that, when energized with acurrent flowing in the windings, a magnetic flux is created in a firstdirection;

the rotor having a plurality of rotor elements, the rotor elements beingspaced from each other and interstitially disposed with the statorelements in an interdigitated manner;

each rotor element being in the form of a circular disk mounted on theshaft, each rotor element comprising a plurality of magneticallyisolated magnetic teeth arranged in an annular portion of the circulardisk;

means for completing a magnetic flux path in a second direction throughthe magnetic teeth of the rotor elements and through correspondinggroups of teeth on successive stator elements.

The present invention further comprises a modular control unit arrangedto individually control electrical energy applied to each winding ofeach stator element, the control unit comprising:

(a) a microprocessor controller, a load sensing means, a rotor angleposition sensor, and a plurality of stator control modules, each controlmodule comprising an electrical switching device connected to a windingof the respective stator element; the microprocessor controller beingresponsive to the load sensing means and to the rotor angle positionsensor to generate control signals to the control modules, each controlmodule being responsive to the control signals to control the flow ofcurrent to the connected winding of the stator element in a pulse-widthcontrol manner,

the flow of current to each winding being turned on at a firstpredetermined rotor angle position and turned off at a secondpredetermined rotor angle position by the control unit in response tocontrol signals from the controller, thus causing the rotor to rotate ata speed responsive to the control signals with a power outputproportional to the load.

In the present invention each control module may further comprisecurrent sensing means to sense current in the windings of the associatedstator element and means to generate a corresponding signal to themicroprocessor controller, the controller being responsive to the signalto compensate for the sensed current and to generate control signals tothe control modules to equalize the current in each phase of the statorwindings.

In the control unit of the present invention, the microprocessorcontroller compares each current signal to a predetermined faultthreshold to detect a winding fault and then causes that control moduleto deenergize one or more windings of the stator element in response tothe detected fault, permitting the motor to continue to operate.Alternatively, the controller may cause the control modules todeenergize all the windings in a stator element is response to adetected fault.

In the present invention the load sensing means may comprise a motorspeed sensor or a torque sensor.

The present invention also comprises a method of operating a unipolarelectrical machine, as described above, in a fault tolerant manner inresponse to a controller,

-   -   the controller being responsive to the load sensing means, the        rotor angle position sensing means, and the signal representing        the sensed winding current, to generate control signals to the        control modules;    -   the controller comparing each current signal to a predetermined        fault threshold to detect a fault in a winding in a stator        element and causing the corresponding control module to        deenergize one or more windings in the stator element in        response to the detected fault,        thereby permitting the motor to continue to operate in the        presence of a winding fault.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 shows an arrangement of components for a motor of the presentinvention;

FIG. 2 shows a representative assembly overview of shaft, rotor disks,stators and housing of the present invention;

FIG. 3 shows a segmented stator construction of the present invention,showing only a portion of the stator windings, the windings surroundingindividual magnetic teeth;

FIG. 4 shows an exemplary arrangement of a stator element constructionof the present invention with the windings surrounding groups ofmagnetic teeth;

FIGS. 5A, B and C show an exemplary arrangement of a rotor elementconstruction of the present invention;

FIG. 6 shows a view with the magnetic teeth of the stator elements andthe magnetic teeth of the rotor elements misaligned;

FIG. 7 shows a view with the magnetic teeth of the stator elements andthe magnetic teeth of the rotor elements aligned; and

FIG. 8 shows a plot of winding current versus rotor position angle for afour-phase switched reluctance motor.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is exemplified by the Figures. As may be seen inFIG. 1, the electrical machine 100 the present invention comprises amotor unit 110 and a modular control unit 160. The motor unit 110 (bestseen in FIG. 2) comprises a housing 120, a stator 130 mounted in thehousing, and a rotor 140, the rotor 140 having an axis 140A therethroughand being supported in the housing 120 by bearings 122 for rotationabout the axis 140A. The housing 120 oprionall may have air vents 124 orheat radiating ribs 126. The stator 130 has a plurality of statorelements 132 in the form of annular disks 132D (best seen in FIGS. 3 and4), spaced apart from each other, each stator element 132 comprising aplurality of magnetically isolated magnetic teeth 136 (FIGS. 3 and 4).

The rotor 140 has a plurality of rotor elements 142 (FIGS. 5A, 5B, 5C)mounted on a shaft 144, the rotor elements 142 spaced from each otherand interstitially disposed with the stator elements 132 in aninterdigitated manner (FIG. 2).

As may best be seen in FIG. 3, in a first arrangement, each of thestator elements 132 has a plurality of electrical windings 134, eachwinding 134 being associated with a magnetic tooth 136 of the statorelement 132. As may best be seen in FIG. 4, in a second arrangement,each of the stator elements 132 has a plurality of electrical windings134, each winding 134 being associated with a group 136G of magneticteeth 136 of the stator element 132, the windings 134 being arrangedsuch that, when energized with a current 134C flowing in the windings134, a magnetic flux M₁ is created in a first direction D₁. It should benoted that some of the stator element components have been omitted sothat the winding 134 may be better seen. Although the Figures illustratean electrical machine having four windings 134-a, 134-b, 134-c, 134-d ineach stator element 132, the machine of the present invention may haveany desired number of windings 134 in each stator element 132.

The magnetic flux path is completed in a second direction D₂ throughcorresponding groups 136G of teeth 134 on successive stator elements132, and through the rotor elements 142, thereby resulting in arotational force F being applied to the rotor elements 142 causing therotor 140 to rotate.

As seen in FIG. 1, the electrical machine 100 further comprises amodular control unit 160 arranged to individually control electricalenergy applied to each winding 134 of each stator element 132. Thecontrol unit 160 comprises a microprocessor controller 162, a loadsensing means 170, a rotor position sensor 176 and a plurality of statorcontrol modules 180.

Each control module 180 comprises a plurality of electrical switchingdevices 182 connected to the respective winding 134 of a stator element132. Suitable electrical switching devices 182 include isolated gatebias transistors (IGBT) and similar devices capable of switching therequired current. The microprocessor controller 162 is responsive to theload sensing means 170 and the rotor position sensor 176 to generatecontrol signals S to the control modules 180. The load sensing means 170may comprise a motor speed sensor 172 or a torque sensor 174. Eachcontrol module 180 is responsive to the control signals S to control theflow of current 134C to the connected winding 134 of stator element 132in a pulse-width control manner by turning on and off the electricalswitching devices 182, thus causing the rotor 140 to rotate at a desiredspeed with a desired power output proportional to the load.

The rotor angle position sensor 176 is typically implemented by anoptical encoder. Rotor shaft position feedback is needed to synchronizethe current flow in each winding 134 with rotor position in order togenerate the desired motoring torque. The rotor angle position is alsoused by the controller 162 to compute actual rotor rotational speed,which is compared with the desired speed and used to control the rotorrotational speed.

Each control module 180 further comprises current sensing means 190 tosense current 134C in the windings 134 of the associated stator element132 and current signaling means 192 to generate a corresponding signal192S to the microprocessor controller 162, the controller beingresponsive to the signal 192S to generate control signal S to thecontrol modules 80, which in turn generate signals 166S1, 166S2, 166S3,166S4 to the switching devices 82 to equalize the current 134C-a,134C-b, 134C-c, 134C-d in each phase of the stator windings 134.

The microprocessor controller 162 compares each current signal 192S to apredetermined fault threshold 164F (schematically represented as beingsupplied by potentiometer 164) to detect a winding fault. When a windingfault is detected the microprocessor controller 162 sends a controlsignal S to that control module 180 which in turn generate signals166S1, 166S2, 166S3, 166S4 to the switching devices 82 to deenergize oneor more windings 134 of stator element 132 in response to the detectedfault, permitting the motor 100 to continue to operate.

The power supplied to the control module 180 is direct current (i.e., isunipolar). The direct current may be supplied by any suitable directcurrent source, such as a DC power supply having a conventionalrectifier, filter and voltage regulation means. The DC power supply maybe fed from a single phase AC powerline source V₁ or from a multiphaseAC source V_(M).

The electrical machine of the present invention may have any desirednumber of windings 134 in each stator element 132. An exemplaryembodiment having four windings (i.e., four phases) 134-a, 134-b, 134-c,134-d is shown. When multiple windings 134 are employed the currentsensing means 190 is arranged to sense current 134C in each individualwinding (phase) 134-a, 134-b, 134-c, 134-d in each stator element 132.Signal generating means 192-a, 192-b, 192-c, 192-d generatecorresponding sensing signals 192S1, 192S2, 192S3, 192S4 to themicroprocessor controller 162. The controller 162 is responsive to eachsensing signal 192S1, 192S2, 192S3, 192S4 to generate control signal S(which may comprise components 166S1, 166S2, 166S3, 166S4) to thecontrol modules 180 to equalize the currents 134C-a, 134C-b, 134C-c,134C-d in each of the stator windings 134-a, 134-b, 134-c, 134-d.

The microprocessor controller 162 compares each current sensing signal192S1, 192S2, 192S3, 192S4 from each winding 134 in each stator element132 to a predetermined fault threshold 164F to detect a winding fault ina stator element 132, and causes the control module 180 connected tothat stator element 132 to deenergize one or more windings 134 of thatstator element 132 in response to the detected fault, permitting themotor 100 to continue to operate.

The microprocessor controller 162 may respond to the sensed current inthe faulted winding 134 in a number of ways. The microprocessorcontroller 162 may increase the current in the other windings 134 ofthat stator element 132 to compensate for the fault. Alternatively, themicroprocessor controller 162 may respond to the sensed current in thefaulted winding 134 to increase the current in the windings 134 of otherstator elements 132 not having the faulted winding 134 to compensate forthe fault. In response to a fault at least two windings 134 in a statorelement 132 may be deenergized, the deenergized windings 134 beingsymmetrically arranged on the stator element 132, so that torqueproduced by that stator element fluctuates in a symmetrical manner. Whenone or more windings in a stator element 132 not containing the faultedwinding 134 are deenergized, those deenergized windings are arranged onthe unfaulted stator element rotationally symmetrical to the faultedwinding 134 on the faulted stator element 132, so that torque producedby the electrical machine remains substantially constant.

A unipolar electrical machine operated as a switched reluctance motor isa singly-excited motor with salient poles 136P on the stator 130 (FIGS.6 and 7) and salient poles 146P on the rotor 140 (FIGS. 6 and 7). Onlythe stator 130 carries windings 134 on the stator elements 132. Therotor 140 has neither windings nor magnets and is built up from a stackof steel laminations. Each stator winding 134 (phase) consists of twoseries connected windings 134X, 134Y (FIG. 3) on diametrically oppositemagnetic teeth 136 to create magnetic poles. Alternatively, statorwinding may consist of windings that are wound around groups 136G ofmagnetic teeth 136 (FIG. 4).

In a switched reluctance motor torque is produced by the tendency of itsmoveable part (i.e., rotor 140) to move to a position where theinductance of the excited winding 134 of the stator element 132 ismaximized. During motor operation, each winding 134 of the statorelement 132 is excited (i.e., current is turned on) when its inductanceis increasing, and unexcited (i.e., current is turned off) when itsinductance is decreasing. The air gap 200 (FIGS. 6 and 7) is at aminimum at the aligned position, the position where a pair of rotorpoles is exactly aligned with a stator pole as seen in FIG. 7, and themagnetic reluctance of the flux flow is at its lowest. The magneticreluctance will be highest at the unaligned position as seen in FIG. 6.Thus, when a winding 134 of the stator element 132 is energized amagnetic pole on the stator is created. If a magnetic tooth 146 on therotor 140 is not aligned with that magnetic stator pole 136P created bythe current in the winding 134 around magnetic tooth 136, the rotor 140will start to move and attempt to align with the stator pole 136P. Ifthe rotor position is known (as sensed by rotor angle position sensor176) the current to successive stator windings 134 may be switched onand off in a controlled manner, causing the rotor 140 to rotate in adesired direction and at a desired speed.

The controller 162 implements conventional switched reluctance motortorque, speed and position control protocols (i.e., current or torquemethod control) with control of control modules 180 connected toindividual windings 134 of the segmented stator elements 132. Combinedwith current sensing inputs 192S the control unit therefore implements aself-regulating variable power output device.

Operation of a Four-Phase Switched Reluctance Motor

Since switched reluctance motors operate on the principle of unipolarcurrent, i.e., the current flows only in one direction in the windings134 regardless of whether positive or negative torque is required. Thisprinciple requires only one switch 182 to be in series with each phasewinding 134. The turning on or off of this switch 182 regulates the flowof current 134C in the phase winding 134. The exemplary four-phaseconstruction, where the phases are identified respectively as: a, b, c,d, when operating, has phase currents 134C-a, 134C-b, 134C-c, 134C-dplotted as shown in FIG. 8 (lines i_(a), i_(b), i_(c), i_(d)respectively).

As seen in FIGS. 3, 4, 6 and 7, there are a plurality of magneticallyisolated magnetic teeth 136 on each of the stator elements 132. Thereare a plurality of electrical windings 134 on each stator element 132.Each of the windings 134 surrounds a different group 136G of magneticteeth 136 to create a magnetic pole 136P. There are also return meansincluding magnetic material for establishing a magnetic flux pathaxially in series through corresponding groups of teeth 136 onsuccessive stator elements 132 and teeth 146 on the interstitial rotorelements 142, and azimuthally in the return means.

The number of teeth 146 on each rotor element 142 may be the same as ordifferent in number from the number of teeth 136 on each stator element132. There are return means including magnetic material for establishinga low reluctance azimuthal flux path and a plurality of conductor pathssurrounding the teeth on each of the rotors. In a preferred embodimentthe stator elements 132 are each in the form of an annular disk 132D.The magnetic teeth 136 of the stator elements 132 are imbedded in theannular disks 132D, and the annular disks 132D are of laminatedconstruction.

FIGS. 5A, 5B and 5C show a preferred embodiment of the rotor elements142, which are each in the form of a circular disk 142D mounted on theshaft 144. In a similar manner the magnetic teeth 146 of the rotorelements 142 are imbedded in the circular disks 142D, and the circulardisks 142D are of laminated construction.

In the unipolar current electrical machine of the present inventionmaterials of construction known and used in conventional designs ofmotors, generators, transformers and the like may be used. In thepresent invention the various control means and electrical componentsfor control may be assembled from electrical components known and usedin conventional electrical control circuits. For example, the loadsensing means 170 (FIG. 1) may be a conventional load or torque sensorand the current sensing means 190 may be a plurality of controltransformers (CT).

In the present invention, the specific geometric relationship of therotor and stator elements according to the present invention is unique.This unique segmented geometry not only provides advantages in theefficiencies of operation, but also results in a surprising ease ofmanufacturing and interchangeability electrical components.

The present invention provides a method of manufacture of a unipolarcurrent electrical machine having a housing, a stator mounted in thehousing, and a rotor, the rotor having a shaft with an axis therethroughand being supported by bearings for rotation about the axis in thehousing, the method comprising the steps of:

(a) selecting from a plurality of individual stator elements the numberof stator elements necessary to produce the desired horsepower orkilowatt rating for the electrical machine wherein stator elements arein the form of annular disks comprising a plurality of magneticallyisolated magnetic teeth, each stator element having thereon a pluralityof electrical windings on each of the stator elements, each windingbeing associated with a group of magnetic teeth of the stator element,the windings being arranged such that, when energized with a currentflowing in the windings, a magnetic flux is created in a firstdirection;

(b) selecting from a plurality of individual rotor elements necessary toproduce the desired horsepower or kilowatt rating for the electricalmachine, wherein the rotor elements are in the form of circular diskscomprising a plurality of magnetically isolated magnetic teeth arrangedin an annular portion of the circular disks;

(c) mounting the selected stator elements in the housing such that theelements are spaced apart from each other;

(d) mounting the selected rotor elements on the shaft such that therotor elements are spaced from each other and interstitially disposedwith the stator elements in an interdigitated manner, so that a magneticflux path in a second direction is completed through correspondinggroups of teeth on the rotor elements and through corresponding groupsof teeth on successive stator elements.

Stator and rotor element combinations according to the present inventionmay be produced in selected sizes, for example 2, 5, 10 or 20horsepower. To build a 40 horsepower motor, for example, one couldselect and stack two 20 horsepower stator rotor combinations, four 10horsepower combinations, eight 5 horsepower combinations, or twenty 2horsepower combinations.

This ability to combine combinations of simple elements or segmentssimplifies the manufacturing process. Motors may be produced in anassembly line operation rather than by custom manufacture. Repairs arealso simplified. For example, a motor may be repaired by simplyreplacing a bad stator or rotor element with a good one. Field repairsmay be possible in many situations. Parts inventories are simplified bythe present invention in that simple stock parts can be manufactured andselected in combinations to produce the desired power output in aparticular electrical machine.

The present invention is a unipolar current electrical machine having anovel arrangement of rotor and stator to produce increased torquecompared to a conventional machine design, yet in a machine that issmaller in size than a conventional machines. It is believed that thisincrease in output/decrease in size is accomplished by passing the samemagnetic flux created by the stator through multiple air gaps andinteracting the magnetic flux at each gap with an adjacent rotorelement, so that the force density is multiplied by the number of gaps.

The present invention is believed to offer the following advantages overconventional designs:

(1) Improved efficiency: The power output is maximized by optimizing thecross-sectional area of magnetic flux interaction between the stator andthe rotor. This results in an increase of magnetic energy transfer andimproved efficiency compared to conventional machines. Due to theincreased magnetic energy transfer, less electrical power will berequired to produce the same amount of mechanical power compared toprior art machines. Unlike prior art designs, the present inventionutilizes independent stator elements and a split motor housing design,allowing individual segments of the stator to be removed and replaced,facilitating easier maintenance.

(2) Enhanced Control Capability: The motor design includes an integralmicroprocessor-based control unit that allows control of each individualwinding of each stator element. This feature allows the motor to run ata constant speed with the optimal horsepower rating for a given load.Motor locked-rotor and in-rush currents are reduced compared toconventional designs. Due to the utilization of the individual statorelement control, the motor can be started at a lower power rating,resulting in further reduction of locked-rotor and in-rush currents.

(3) Reductions in Size, Weight and Cost: The machine of the presentinvention can be smaller than conventional motors of an equivalenthorsepower rating. The segmented design permits the manufacture ofstandardized modules of stator elements and rotor elements. By varyingthe number of modules which are assembled, machines of various sizes andpower ratings may be achieved. The manufacture of interchangeablemodules can reduce costs.

Those skilled in the art, having the benefits of the teachings of thepresent invention as hereinabove set forth, may effect numerousmodifications thereto. Such modifications are to be construed as lyingwithin the contemplation of the present invention, as defined by theappended claims.

1. An unipolar current electrical machine having a housing, a statormounted in the housing, and a rotor, the rotor having a shaft with anaxis therethrough and being supported by bearings for rotation about theaxis in the housing, comprising: the stator having a plurality of statorelements each in the form of an annular disk, spaced apart from eachother, each stator element comprising a plurality of magneticallyisolated magnetic teeth; a plurality of electrical windings on each ofthe stator elements, each winding being associated with a group ofmagnetic teeth of the stator element, each group of magnetic teeth beingarranged at a predetermined angular position with respect to an adjacentgroup of magnetic teeth, each of the windings being arranged such that,when energized with a current flowing in the windings, a magnetic fluxis created in a first direction; the rotor having a plurality of rotorelements, the rotor elements being spaced from each other andinterstitially disposed with the stator elements in an interdigitatedmanner; each rotor element being in the form of a circular disk mountedon the shaft, each rotor element comprising a plurality of magneticallyisolated magnetic teeth arranged in a annular portion of the circulardisk; means for completing a magnetic flux path in a second directionthrough the magnetic teeth of the rotor elements and throughcorresponding groups of teeth on successive stator elements. a modularcontrol unit arranged to individually control electrical energy appliedto each winding of each stator element, the control unit comprising: amicroprocessor controller, a load sensing means, a rotor angle positionsensor, and a plurality of stator control modules, each associated witha respective stator element, each stator control module comprising aplurality of electrical switching devices, each connected to a windingof the respective stator element; the microprocessor controller beingresponsive to the load sensing means and the rotor angle position sensorto generate control signals to the control modules; each control modulebeing responsive to the control signals to control the flow of currentto each connected winding of the stator element in a pulse-width controlmanner, the flow of current to each winding being turned on at a firstpredetermined rotor angle position and turned off at a secondpredetermined rotor angle position by the control unit in response tocontrol signals from the controller load sensing means and the currentsensing means, thus causing the rotor to rotate at a speed androtational direction responsive to the control signals, with a poweroutput proportional to the load.
 2. The electrical machine of claim 1,each control module further comprising current sensing means to sensecurrent in each winding of the associated stator element and means togenerate a corresponding signal to the microprocessor controller, thecontroller being responsive to the signal to compensate for the sensedcurrent.
 3. The electrical machine of claim 2, the microprocessorcontroller comparing each current signal to a predetermined faultthreshold to detect a winding fault in a stator element and to causethat control module to deenergize one or more windings in the statorelement in response to the detected fault, permitting the motor tocontinue to operate.
 4. The electrical machine of claim 3, themicroprocessor controller, in response to the detected fault, to causethe control modules corresponding to other stator elements not havingthe winding fault to increase current in the windings of those statorelements, permitting the motor to continue to operate at the desiredspeed and power output.
 5. The electrical machine of claim 2, whereinthe unipolar current is supplied from a control unit having multiplephase outputs, each output connected to a corresponding winding, thecurrent sensing means being arranged to sense current in each individualwinding in each stator element winding, means to generate acorresponding sensed current signal to the microprocessor controller,the controller being responsive to the sensed current signals togenerate control signals to the control modules to equalize the currentin each of the stator windings.
 6. The electrical machine of claim 5,the microprocessor controller comparing each current sensing signal fromeach phase in the winding of each stator element to a predeterminedfault threshold to detect a winding fault in a stator element, and causethe control module connected to that stator element to deenergize thestator element in response to the detected fault, permitting the motorto continue to operate.
 7. The electrical machine of claim 1, the loadsensing means comprising a motor speed sensor.
 8. The electrical machineof claim 1, the load sensing means comprising a torque sensor.
 9. Theelectrical machine of claim 1, the rotor position sensing meanscomprising a rotary encoder.
 10. The electrical machine of claim 1,wherein the electrical machine is used as a motor.
 11. A method ofmanufacture of a unipolar current electrical machine having a housing, astator mounted in the housing, and a rotor, the rotor having a shaftwith an axis therethrough and being supported by bearings for rotationabout the axis in the housing, the method comprising the steps of: (a)selecting from a plurality of individual stator elements the number ofstator elements necessary to produce the desired horsepower or kilowattrating for the electrical machine wherein stator elements are in theform of annular disks comprising a plurality of magnetically isolatedmagnetic teeth, each stator element having thereon a plurality ofelectrical windings on each of the stator elements, each winding beingassociated with a group of magnetic teeth of the stator element, thewindings being arranged such that, when energized with a current flowingin the windings, a magnetic flux is created in a first direction; (b)selecting from a plurality of individual rotor elements necessary toproduce the desired horsepower or kilowatt rating for the electricalmachine, wherein the rotor elements are in the form of circular diskscomprising a plurality of magnetically isolated magnetic teeth arrangedin an annular portion of the circular disks; (c) mounting the selectedstator elements in the housing such that the elements are spaced apartfrom each other; (d) mounting the selected rotor elements on the shaftsuch that the rotor elements are spaced from each other andinterstitially disposed with the stator elements in an interdigitatedmanner, so that a magnetic flux path in a second direction is completedthrough corresponding groups of teeth on the rotor elements and throughcorresponding groups of teeth on successive stator elements.
 12. Amethod of operating an electrical machine in a fault tolerant manner inresponse to a controller, the electrical machine comprising: a housing,a stator mounted in the housing, and a rotor, the rotor having a shaftwith an axis therethrough and being supported by bearings for rotationabout the axis in the housing, the stator comprising: a plurality ofstator elements in the form of annular disks, spaced apart from eachother, each stator element comprising a plurality of magneticallyisolated magnetic teeth, a plurality of electrical windings on each ofthe stator elements, each winding being associated with a group ofmagnetic teeth of the stator element, each group of magnetic teeth beingarranged at a predetermined angular position with respect to an adjacentgroup of magnetic teeth, each of the windings being arranged such that,when energized with a current flowing in the windings, a magnetic fluxis created in a first direction; the rotor comprising: a plurality ofrotor elements, the rotor elements being spaced from each other andinterstitially disposed with the stator elements in an interdigitatedmanner; each rotor element being in the form of a circular disk mountedon the shaft, each rotor element comprising a plurality of magneticallyisolated magnetic teeth arranged in an annular portion of the circulardisk; means for completing a magnetic flux path in a second directionthrough the magnetic teeth of the rotor elements and throughcorresponding groups of teeth on successive stator elements; a modularcontrol unit arranged to individually control electrical energy appliedto each winding of each stator element, the control unit comprising: amicroprocessor controller, a load sensing means, a rotor angle positionsensor, and a plurality of stator control modules; each stator controlmodule comprising an electrical switching device connected to a windingof the respective stator element and current sensing means to sensecurrent in each winding of the associated stator element and means togenerate a corresponding signal to the microprocessor controller; themicroprocessor controller being responsive to the load sensing means,the rotor angle position sensor, and the signal representing the sensedwinding current, to generate control signals to the control modules; themicroprocessor controller comparing each current signal to apredetermined fault threshold to detect a fault in a winding in a statorelement and causing the corresponding control module to deenergize oneor more windings in the stator element in response to the detectedfault, thereby permitting the motor to continue to operate in thepresence of a winding fault.
 13. The method of claim 12, furthercomprising the microprocessor controller being responsive to the sensedcurrent in the faulted winding to increase the current in the otherwindings of that stator element to compensate for the fault.
 14. Themethod of claim 12, further comprising the microprocessor controllerbeing responsive to the sensed current in the faulted winding toincrease the current in the windings of other stator elements tocompensate for the fault.
 15. The method of claim 12, wherein at leasttwo windings in a stator element are deenergized, the deenergizedwindings being symmetrically arranged on the stator element, so thattorque produced by that stator element fluctuates in a symmetricalmanner.
 16. The method of claim 12, wherein one or more windings in astator element not containing the faulted winding are deenergized, thedeenergized windings being arranged on the unfaulted stator elementrotationally symmetrical to the faulted winding on the faulted statorelement, so that torque produced by the electrical machine remainssubstantially constant.